The logback manual chapter 7 says:

package org.slf4j;

public class MDC {
...
//Returns the keys in the MDC as a Set. The returned value can be null.
public static Set<String> getKeys();
}

But the method getKeys() is not part of the released slf4j API (versions 1.4.3 and 1.5.0).

This method exists only within the LogbackMDCAdapter.